The present invention relates to the fabrication of photodetectors. In particular, the present invention is a method of fabricating a GaP UV sensitive photodiode.
The first consideration in the development of a solid state photodetector is matching the band gap of the detector material to the wavelength region of interest. In the case of solid state ultraviolet detectors, this has presented a serious difficulty since a wide band gap material is required. In general, the wide band gap semiconductor materials have received very little attention and are at a very early stage of development. This has, of course, hindered the development of ultraviolet sensitive solid state sensors.
Of the well-developed semiconductor materials, gallium phosphide (GaP) has the widest band gap (an indirect band gap of 2.24 eV and a direct band gap of 2.8 eV). GaP has found wide use as a material for light emitting diodes, which accounts for its relatively well-developed materials technology in comparison to other wide band gap semiconductor materials.
Because the 2.24 eV indirect band gap of GaP corresponds to a wavelength of 0.55 microns, it has been recognized that visible wavelength photodetectors could be made from GaP. Compared to CdS and other visible wavelength detector materials, however, much less work has been done in investigating the photodetector properties of GaP. Among the references which describe photoconductive properties of GaP in the visible spectrum are G. W. Allen and R. J. Cherry, J. Phys. Chem. Solids, 23, 503 (1962); H. G. Griemmeiss and H. Scholz, Philips Res. Rev., 148, 715 (1966); D. L. Bowman, J. Appl. Phys., 38, 568 (1967); D. F. Nelson et al, Phys. Rev., 135, A1399 (1964); and U.S. Pat. Nos. 3,261,080; 3,412,252; and 3,915,754.
Petersen and Schulze, in U.S. Pat. No. 3,976,872 first reported that under some conditions a usable photoresponse to ultraviolet wavelengths can also be obtained from GaP. The specific device with which Petersen and Schulze made this initial discovery was a photoconductive GaP detector having an asgrown surface.
Subsequently, a GaP Schottky barrier device which exhibited ultraviolet photoresponse was reported in a Russian technical journal: Fiz. Tekh. Poluporev., 8, 410-413 (1974). A Schottky barrier device is a metal-to-semiconductor rectifying contact which forms a potential barrier at the interface between the metal and the semiconductor and creates an internal directed surface field within the semiconductor.
Except for the Petersen and Schulze patent and the Russian technical journal article, there have been no reports of usable ultraviolet photoresponse in GaP. In particular, prior to the present invention, there has been no report of a PN junction GaP photodiode exhibiting usable photoresponse to ultraviolet radiation. A PN junction GaP photodiode exhibiting ultraviolet photoresponse would have several advantages over the specific devices described by Petersen and Schulze, and the Russian technical journal article. First a PN junction device may utilize developed PN junction formation technology. Second a PN junction device may have less stringent surface preparation requirements. Third, a PN junction device has a potentially higher quantum efficiency than a Schottky barrier device because of absorption in the Schottky barrier contact and the potential valley just below the metal-semiconductor interface. Fourth, a PN junction device may be capable of avalanche photodiode operation. Despite these potential advantages, no PN junction GaP photodiode exhibiting ultraviolet photoresponse has been developed.
In recent years, numerous techniques have been developed and investigated for improving the response of visible wavelength solar cells. One technique which has been investigated is the use of impurity gradients in PN junctions to create internal electric field gradients. These internal electric field gradients have improved the quantum efficiency of solar cells by sweeping photogenerated minority carriers away from the surface toward the PN junction. Description of internal electric field gradients in PN junction solar cells may be found in "Photo Effect on Diffused PN Junctions with Internal Field Gradients", I.R.E. Trans. on Electron Devices, ED-7, 242 (1960); B. Ellis and T. S. Moss, "Calculated Efficiencies of Practical GaAs and Si Solar Cells Including the Effect of Built-In Electric Fields", Solid State Electronics, 13, 1 (1970); K. V. Vaidyanathan and G. H. Walker, "The Effect of Be.sup.+ Ion Implanted Exponential and Uniform Impurity Profiles on the Electrical Characteristics of GaAs Cells"; and U.S. Pat. No. 4,001,864 by J. F. Gibbons. No ultraviolet response was reported for any of the devices described in these references. In addition, there was no discussion in these references of the use of GaP or the effect which a PN junction with an internal electric field gradient may have on the photoresponse of GaP.